We have directly measured
the mass diffusion coefficient D of 3He
in superfluid 4He at temperatures
lower than were previously possible. The experimental technique
utilizes scintillation light produced when neutrons react
with 3He nuclei,
and allows measurement of the 3He density integrated along
the trajectory of a well-defined neutron beam. By measuring
the change in 3He density near a heater as a function
of applied heat current,
we are able to infer values for D
with 20% accuracy.
At temperatures below 0.7 K and for concentrations
of order 10-4, we find cm2/s, in
agreement with a theoretical approximation.

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